To leave port is least problematic (3,4)
I believe the answer is:
set sail
'to leave port' is the definition.
(I've seen this before)
'is least problematic' is the wordplay.
'problematic' indicates anagramming the letters.
'is'+'least'='isleast'
'isleast' anagrammed gives 'SET SAIL'.
(Other definitions for set sail that I've seen before include "Left port" , "Begin a voyage" , "Embark on a voyage by ship" , "Leave for (sea) voyage" , "have started journey" .)